Automatic color crt tracking tester and method of testing

ABSTRACT

Tracking of the three guns of a color cathode ray tube is tested to determine if the tracking is within allowable limits by a circuit which stores voltages proportional to the emission currents of the guns under standard operating conditions and compares such voltages to provide an indication if such voltages differ from one another by more than a predetermined amount.

United States Patent Baum , Aug. 29, 1972 AUTOMATIC COLOR CRT TRACKINGTESTER AND METHOD OF TESTING Primary ExaminerRudolph V. Rolinec [72]Inventor: Robert E. Baum, Dell Rapids, S.

Assistant Exammer-Dawd M. Carter Attorney-Fidler, Patnaude & Lazo [73]Assignee: Sencore, Inc, S1oux Falls, S. Dak. [22] Filed: June 1, 1971[57] ABSTRACT [21] Appl. No.: 148,628 Tracking of the three guns of acolor cathode ray tube is tested to determine if the tracking is withinallowa- 52 US. Cl. ..324/20 CR, 307/279,17s/5.4 TE ble by a circuitwhich "wages 51 Int. Cl. ..Glr 31/22, H04n 9/20 emissim currents theguns under Stan 58 Field of Search ..324/ CR; l78/5.4 TE; dard operatingconditions and compares such g s 307 279 to provide an indication ifsuch voltages differ from one another by more than a predeterminedamount.

[56] References Cited 9 Cl 1 Drawing UNITED STATES PATENTS v 3,270,l8/1966 Kelly ..l78/5.4 TE

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AUTOMATIC COLOR CRT TRACKING TESTER AND METHOD OF TESTING The presentinvention relates to a novel method and circuit for testing the trackingcharacteristics of a color cathode ray tube, and it relates moreparticularly to a circuit for automatically checking the trackingcharacteristics and indicating whether such characteristics are withinallowable limits.

in order to provide a satisfactory, visual display on the screen of acolor cathode ray tube of the multiple gun type, it is necessary for allof the guns to have similar emission characteristics, i.e., the emissioncurrents from the individual guns must not appreciably vary relative toone another under normal operating conditions. The emissions from theseparate guns are tested by what is known as an-emission check whereinthe several guns are each biased close to cut-off and then the bias onall tubes is reduced by an identical amount to provide a substantialemission current from each of the guns. For proper tracking most cathoderay tube manufactures recommend that the emission current from thehighest current gun be no greater than one and one-half times theemission current from the lowest current gun. Experience also shows thata greater emission current ratio results in unacceptably poor trackingand correspondingly poor picture display.

ln accordance with the prior art techniques for checking tracking it wasnecessary to initially determine .which gun had the highestemissioncurrent and which gun had the lowest emission current and thento compare the emission currents from thosetwo guns. This was a timeconsuming multiple step method.

An object of the present invention is to provide a new and improvedmethod and circuit for checking the tracking characteristics of amultiple gun color cathode ray tube.

., Another object of this invention is to provide a circuit forautomatically checking the tracking characteristics of a color cathoderay tube.

A further object of this invention is to provide a circuit usable withsubstantially all types of present-day color cathode ray tubes fordetermining whether the tracking characteristics thereof are withinstandard, acceptable limits.

Briefly, the above and further objects may be realized in accordancewith the present invention by means of a circuit which stores signalsproportional to the maximum emission current of each gun during theemission check, and automatically compares the values of such signals toprovide an indication if the ratio of the maximum signal to the minimumsignal exceeds a predetermined value.

Further objects and advantages and a better understanding of theinvention may be had from the following detailed description taken inconnection with the accompanying drawing, wherein:

The single F lGURE is a schematic diagram of an automatic trackingtesting circuit embodying the present invention.

Referring now to the drawing, the red, green and blue electron guns of acolor cathode ray tube are generally indicated at 10, 11 and 12respectively. As shown, each of the electron guns l0, l1 and 12 includesa cathode, a control grid G1 and a screen grid G2. When testing theoperation of the electron guns of a color cathode ray tube the standardpractice is to apply a high negative bias on the control grids G1, tothen adjust the bias on the screen grids to that value where theemission currents are minimal, and then remove the bias on the controlgrids and measure the emission current from each of the cathodes. 1f thehigh voltage anode of the tube is not energized during this test a metermay be connected in series with the screen grids to measure the emissioncurrent to indicate if the emission current is sufficient for asatisfactory picture display on the face of the cathode ray tube.

In order to perform the just described emission test using the circuitof the present invention, the socket of the color cathode ray tube beingtested is connected to the illustrated test circuit by means of suitableplug connections not shown so that the cathodes of the three guns l0, l1and 12 are respectively connected to switch contacts l3, l4 and 15 of agun selector switch wafer 16 having a wiper l7 selectively engageablewith the switch contacts 13, 14 and 15. The control grids G1 of theelectron guns 10, 11 and 12 are respectively connected to switchcontacts 19, 20 and 21 of a gun selector switch wafer 22 having a wiper23 selectively connectible to the contacts 19, 20 and 21. Similarly, thescreen grids G2 of the electron guns 10, 11 and 12 are respectivelyconnected to switch contacts 25, 26 and 27 of a gun selector switchwafer 28 having a wiper 29 selectively connectable to the contacts 25,26 and 27. As shown in the drawings, the wipers 17, 23 and 29 of the gunselector switch are ganged together so that in any one of the threepositions the wipers 17, 23 and 29 are all connected to the elements ofthe same electron gun. in the illustrated position the selector switchis in the red gun selecting position and these wipers are respectivelyconnected to the cathode, control grid G1 and screen grid G2 of the redgun 10.

The wiper 23 of the control grid selector wafer is connected to thewiper 31 of a switch wafer 32 of a function selector switch alsoincluding a wiper 33 and a wiper 34, all of the wipers 31, 33 and 34being ganged together as illustrated. A source of 50 volts negative DCis connected to a contact element 35 which is engaged by the wiper 31when the function selector switch is in a screen bias adjust positionprior to the emission check. With the wiper 31 in contact with theswitch contact 35, the wipers 33 and 34 are respectively engaging thecontact elements 37 nd 38 associated therewith. It maybe seen that inthat position the control grids G1 are conditioned for connection to the50 volt negative bias source through the selector switch wafer 22. Atthis same time an ammeter 40 connected between the wipers 33 and 34 isconnected to the wiper 29 of the screen grid selector wafer 28 and tothe wiper 41 of a selector switch wafer 42 which is used to coupleadjustable screen grid bias voltages to the screen grids.

The switch wafer 42 includesa plurality of contacts 43, I

44 and 45. The wiper 41 is ganged to the other wipers 17, 23 and 29 ofthe gun selector switch.

The adjustable positive bias voltage for the screen grids G2 is providedby a plurality of potentiometers 47, 48 and 49 connected across outputterminals of a DC power supply (not shown). As indicated on the. drawingthe potentiometers are connected between a first terminal having avoltage value of 380 volts and a second terminal having a voltage valueof 24 volts. The

wiper of the potentiometer 47 is connected to the contact element 43,the wiper of the potentiometer 48 is connected to the contact element44, and the wiper element of the potentiometer 49 is connected to thecontact element 45. it may thus be seen that the potentiometer 47 isused to adjust the bias on the screen grid G2 of the red gun, thepotentiometer 48 is used to adjust the bias on the screen grid G2 of thegreen gun and the potentiometer49 is used to adjust the bias on thescreen grid G2 of the blue gun.

In order to perform an emission test to determine if the electronemission from each of the guns 10, l 1 and 12 is satisfactory, thefunction switch is first placed in the screen grid adjust positionwherein the wipers 31, 33 and 34 are respectively connected to thecontact elements 35, 37 and 38 whereby the meter 40 is connected betweenthe wipers 29 and 41 and the 50 volt negative bias source is connectedto the wiper 23. With the gun selector switch in the illustratedposition, i.e., connected to the red gun 10, the control grid G1 of thered gun is biased at 50 volts negative and the voltage from the wiper ofthe potentiometer 47 is connected through the switch wafer 42, a diode50 and the switch wafer 28 to the screen grid G2 of the red gun. Thediode 50 is for meter protection and becomes conductive when the needleof the meter 40 approaches full scale, thereby to keep the meter frompeg ging. The meter 40 is in series with the screen grid G2 of the redgun and the potentiometer 47 is now adjusted until current just beginsto flow through the screen grid G2 and thus through the meter 40. Theselector switch is then moved to the green control grid adjust positionand the potentiometer 48 is adjusted to provide a bias on the screengrid G2 of the green gun 11 to just cause a minimal current flow thereinas indicated by the meter 40. The selector switch is then placed in theblue position and the potentiometer 49 is adjusted to provide minimalcurrent flow through the screen grid G2 of the blue gun.

After the potentiometers 47, 48 and 49 have thus been set to theso-called cut-off position for the respective guns, the program switchis movedto the illustrated emission check position thereby disconnectingthe negative bias voltage from the wiper 23 of the control grid selectorwafer 22. It will be noted that the meter 40 is still connected betweenthe wiper 41 and the wiper 29 by means of a double pole switch 52 havinga pair of arms 53 and 54 respectively connected to contact elements 55and 56 of the function switch. Contact elements 58 and 59 of the switch52 are respectively connected to the wiper 41 and to the wiper 29. Theswitch 52 is preferably a push button switch spring biased in theillustrated position wherein the meter 40 is connected in circuit withthe screen grids, so that upon completion of the emission check thebutton is merely pushed to check the tracking.

With the selector switch in the illustrated position to check the redgun 10, the control grid G1 of the red gun is connected to the cathodethereof by a resistor 60, the purpose of which is to protect the controlgrid G1 and to maintain an essentially zero bias voltage thereonrelative to the associated cathode. The cathode is connected through theswitch wafer 16 and through a resistor 61 to a zero reference bus 62.The cathode is also connected through a resistor 63 to the wiper 64 of aswitch wafer 65 of the selector switch. Accordingly, the wiper 64 isganged to the wipers 17,23, 29 and 41. The wafer 65 further includes aplurality of contacts elements 66, 67 and 68.

In the red check position, the wiper 64 is connected to the contactelement 66 which is in turn connected to the gate element of a fieldeffect transistor (F ET) 70. It will be apparent that the voltagedeveloped across the resistor 61 is proportional to the electronemission from the cathode of the red gun 10 and this voltage is coupledthrough the resistor 63 to the gate of the F ET 70. This voltage is alsodeveloped across a capacitor 71 connected between the gate of the FETand the zero reference bus 62.

The PET is connected in a source follower configura- I tion whereby thedrain electrode is connected to the 24 volt bias source and the sourceelectrode is connected to the zero reference bus 62 through a pair ofserially connected resistors 72 and 73 connected in a voltage dividerarrangement. I The junction between the resistors 72 and 73 is connectedthrough a diode 74 to a conductor 75 connected to a contact element 76of the switch 52. The junction between the source electrode of the FET70 and the resistor 72 is connected through a diode 77 to a conductor 78connected to'the contact element 79 of the switch 52. It may thus beseen that the maximum voltage developed across the resistor 61 with theselector switch in the red emission check position is stored by thecapacitor 71 and, moreover, appears across the resistors 72 and 73. Atthe same time this voltage is developed across the resistor 61, thecurrent through the screen grid G2 of the red gun 10 is measured andindicated by the meter 40 so that the personmaking the test may observefrom the meter reading whether the red gun 10 has a sufficient electronemission to provide a satisfactory picture display on the face of thetube. It will be noted that the screen grid current is measured by themeter 40 and the voltage developed across the cathode resistor 61 isstored on the capacitor 71.

Having thus checked the emission of the red gun and stored a signalproportional thereto, the gun selector switch is moved to the green gunchecking position. In this position the cathode of the green gun isconnected through the contact element 14 and the wiper 17 to thejunction between the resistors 61 and 63 whereby the voltage developedacross the resistor 61 is proportional to the emission current from thegreen cathode. This voltage is coupled through the resistor 63 and thewiper 64 to the contact element 67 which is connected to the gateelectrode of a field effect transistor (FET) 80 and developed across acapacitor 81 connected between the gate of the F ET 80 and the zeroreference bus 62. The drain electrode of the FET 80 is connected to the24 volt bias source and the source electrode is connected through a pairof serially connected resistors 82 and 83 to the zero reference bus 62.The junction between the resistors 82 and 83 is connected through adiode 84 to the conductor 75 and the junction between the resistor 82and the source electrode of the FET 80 is connected through a diode 85to the conductor 78. Accordingly, the voltage stored on the capacitor 81is proportional to the emission current from the green cathode wherebythe voltage developed across the source resistor 82 and coupled to theconductors 75 and 78 through the diodes 84 and 85 is also proportionalthereto.

With the selector switch in the green gun checking position the meter 40indicates the current through the screen grid G2 of the green gun andthe person performing the test may thus determine if such emission issufficient to provide a satisfactory picture display on the face of thetube. Upon completion of the emission check on the green gun the gunselector switch is then moved to the blue gun checking position whereinthe cathode of the blue gun 12 is connected through the contact 15 andthe wiper 17 to the junction between the resistors 61 and 63, and thevoltage developed across the cathode resistor 61 is proportional to theemission current of the blue gun. This voltage is coupled through theresistor 63 and the wiper 64 of the switch sector 65 to the contactelement 68 connected to the gate electrode of a field effect transistor(F ET) 90. This voltage is thus stored on a capacitor 91 connectedbetween the gate electrode of the FET 90 and the zero reference bus 62.A pair of resistors 92 and 93 are serially connected between the sourceelectrode of the FET 90 and the zero reference bus 62 and the drainelectrode of the FET 90 is connected to the positive 24 volt biassource. The junction between the resistors 92 and 93 is connectedthrough a diode 94 to the conductor 75 and the junction between theresistor 92 and the source electrode is connected through a diode 95 tothe conductor 78. At this same time the control grid of the blue gun isconnected to the cathode thereof by the resistor 60 so as to beessentially at zero bias and the meter 40 is connected in circuit withthe green grid G2 of the blue gun so as to indicate the currenttherethrough.

It may thus be seen that upon completion of the emission check on eachof the three electron guns of a color cathode ray tube, voltages whichare respectively proportional to the maximum emission currents from eachof the three guns during the emission check are stored on the capacitors71, 81 and 91. In order to determine the tracking characteristics of thecathode ray tube using the circuit of the present invention, it ismerely necessary to actuate the switch 52 to the automatic trackingchecking position wherein the wipers 53 and 54 are connectedrespectively to the contact elements 76 and 79. If the meter 40 does notindicate that a voltage exceeding a small predetermined value existsbetween the conductors 75 and 78, it is known that the emission ratiobetween the highest current emitting gun and the lowest current emittinggun is less than a predetermined value and satisfactory tracking 'isprovided by the tube being tested. lf, on the other hand, the meterreads a value greater than the predetermined value, it is known that thetracking ratio between the guns of the tube under test exceeds thatvalue at which a satisfactory picture display will be provided. Thesmall predetermined voltage value may be indicated on the meterscale andis necessary to compensate for voltage drops across the diodes.

in order to provide the aforementioned current reading when the trackingratio exceeds 1.5, the values of the resistors 72, 82 and 92; and 73, 83and 93 are selected such that when the voltage across any of the pairsof resistors 72, 83; 82, 83 or 92, 93 is 1.5 times greater than thevoltage developed across either of the other two pairs of resistors, avoltage sufficient to actuate the meter 40 is provided between theconductors and 78. In a reduction to practice of the present inventionthe resistors 72, 82 and 92 each had a value of 2.7 kilohms and theresistors 73, 83 and 93 each had a value of 6 kilohms.

To understand the operation of this automatic tracking checking circuitlet it be assumed that the voltage stored across the capacitor 71 duringthe emission check of the red gun has a value of E. Accordingly, thisvalue of E volts appears at the source electrode of the FET 70. Let itbe assumed that the tracking characteristics of the tube-under test areunsatisfactory and that the green gun has an emission current such thatthe voltage stored on the capacitor 81 is 1.5 E volts. This same valueof 1.5 E volts thus appears on the source electrode of the F ET 80.Since thediodes 77, 85 and 95 are polarized to conduct current to thesource electrodes of the FETs 70, 80 and the voltage developed on theconductor 78 on the negative terminal of a meter protection diode 97connected between the conductors 75 and 78 will be the minimum voltageappearing on any of the sourceelectrodes of the FETs 70,80

and 90. As we have assumed, the emission current from the red gun is thelowest and therefore the voltage of E volts appears on the conductor 78.The voltage appearing between the resistors 82 and 83 is equal to 6/8.7l.5 E volts or 1.03 E volts. Accordingly, a voltage of 0.03 E voltsappears between the conductors 75 and 78 which, when the switch 52 ispositioned in the automatic tracking selection position, is coupledacross the terminals of the meter 40 which will read up scale an amountexceeding the predetermined value. The value is preferably marked on thescale of the meter so that when the meter reads an amount equal to orgreater than such value the tracking is excessive.

It may thus be seen that with the circuit of the present inventionvoltages are stored on the capacitors 71, 81 and 91 proportional to theemission currents thereof during the check of the emissioncharacteristics of each of the guns of a multiple gun color cathode raytube, and a voltage equal to that developed by the lowest emitting gunis coupled to the conductor 78 and a voltage equal to the 66.67 timesthe voltage proportional to the emission from the highest emitting gunis coupled to the conductor 75. If the voltage on the conductor 75exceeds that on the conductor 78, when the switch52 is moved into thetracking check position the meter 40 will read up scale an amountindicating that one of the guns has an emission current which is atleast 1.5 times greater than the emission current of one of the otherguns. The time consuming tracking test of the prior art is thusobviated.

Although the present invention is not limited to the use of anyparticular circuit parameters, it has been found that the followingparameters provide a satisfactory circuit for indicating when thetracking exceeds 1.5:

Resistors 72, 82, 92 2.0 Kohms (2%) Resistors 73, 83, 93 6.7 Kohms (1%)Resistor 61 6.8 Kohms (5%) Resistor 63 100 Kohms Capacitors 71, 81, 91.22 pf FETs 70, 80, 90 2N5457 Diodes 74, 77, 84, 85, 94, 95 |N 34 Diodes50, 97 1N4l48 While the present invention has been described inconnection with particular embodiments thereof, it will be understoodthat those skilled in the art may make many changes and modificationswithout departing from the true spirit and scope thereof. Accordingly,the appended claims are intended to cover all such changes andmodifications as fall within the true spirit and scope of the presentinvention.

What is claimed is:

l. A method of testing the emission and tracking characteristics of amultiple gun cathode ray tube, comprising the steps of sequentiallymeasuring the emission current from each gun of said tube underpredetermined operating conditions and simultaneously storing a voltageproportional to said current, and then comparing the stored voltagesfrom said guns and indicating if the ratio of the highest to the lowestof said voltages exceeds a predetermined value.

2. A method according to claim 1 wherein said emission currents aremeasured by a meter connected in series with the screen grids of saidguns, and

said voltages are derived from resistance means connected in the cathodecircuits of said tube.

3. An automatic tracking tester for a color cathode ray tube having aplurality of electron guns each including a control grid, a screen gridand a cathode, comprismg a plurality of voltage dividers,

means for passing through said dividers currents proportional to therespective cathode emission currents from each of said guns,

said dividers each having a first voltage output proportional to thecurrent passed therethrough and a second voltage output equal to apredetermined percentage thereof,

an ammeter, and

a plurality of unidirectional impedance means connected between saidfirst and second outputs and said ammeter for coupling the lowest of thevoltages appearing at said first outputs to one terminal of said ammeterand the highest of the voltages appearing at said second outputs to theother terminal of said ammeter. 4. An automatic tracking testeraccording to claim 3, comprising means for selectively energizing saidelectron guns, means for developing voltages proportionalf to thecathode emission currents from said guns, and a plurality of storagedevices for storing said voltages developed by said last named means. 5.An automatic tracking tester according to claim 4 comprising gunselector switch means for selectively connecting a positive voltage tothe screen grids of said guns and selectively connecting said storagedevices to the corresponding gun. 6. An automatic tracking testeraccording to claim 4 comprising switch means having a first switchingposition for connecting a negative bias voltage to the control grids ofsaid guns and a second switching position disconnecting said biasvoltage from said control grids.

7. An automatic tracking tester according to claim 6 sQV ii cii meansfor selectively connecting said unidirectional impedance devices to saidammeter only when said negative bias voltage is disconnected from saidcontrol grids.

8. An automatic tracking tester according to claim 4 comprising aplurality of field effect transistors, said voltage dividers areconnected to the source electrodes of said transistors, and said storagedevices are capacitors connected to the gate electrodes of saidtransistors. 9. An automatic tracking tester according to claim 8wherein said means for developing voltages comprises,

resistance means selectively connectible in series with the cathodes ofsaid guns.

1. A method of testing the emission and tracking characteristics of amultiple gun cathode ray tube, comprising the steps of sequentiallymeasuring the emission current from each gun of said tube underpredetermined operating conditions and simultaneously storing a voltageproportional to said current, and then comparing the stored voltagesfrom said guns and indicating if the ratio of the highest to the lowestof said voltages exceeds a predetermined value.
 2. A method according toclaim 1 wherein said emission currents are measured by a meter connectedin series with the screen grids of said guns, and said voltages arederived from resistance means connected in the cathode circuits of saidtube.
 3. An automatic tracking tester for a color cathode ray tubehaving a plurality of electron guns each including a control grid, ascreen grid and a cathode, comprising a plurality of voltage dividers,means for passing through said dividers currents proportional to therespective cathode emission currents from each of said guns, saiddividers each having a first voltage output proportional to the currentpassed therethrough and a second voltage output equal to a predeterminedpercentage thereof, an ammeter, and a plurality of unidirectionalimpedance means connected between said first and second outputs and saidammeter for coupling the lowest of the voltages appearing at said firstoutputs to one terminal of said ammeter and the highest of the voltagesappearing at said second outputs to the other terminal of said ammeter.4. An automatic tracking tester according to claim 3, comprising meansfor selectively energizing said electron guns, means for developingvoltages proportional to the cathode emission currents from said guns,and a plurality of storage devices for storing said voltages developedby said last named means.
 5. An automatic tracking tester according toclaim 4 comprising gun selector switch means for selectively connectinga positive voltage to the screen grids of said guns and selectivelyconnecting said storage devices to the corresponding gun.
 6. Anautomatic tracking tester according tO claim 4 comprising switch meanshaving a first switching position for connecting a negative bias voltageto the control grids of said guns and a second switching positiondisconnecting said bias voltage from said control grids.
 7. An automatictracking tester according to claim 6 comprising switch means forselectively connecting said unidirectional impedance devices to saidammeter only when said negative bias voltage is disconnected from saidcontrol grids.
 8. An automatic tracking tester according to claim 4comprising a plurality of field effect transistors, said voltagedividers are connected to the source electrodes of said transistors, andsaid storage devices are capacitors connected to the gate electrodes ofsaid transistors.
 9. An automatic tracking tester according to claim 8wherein said means for developing voltages comprises resistance meansselectively connectible in series with the cathodes of said guns.